[ENHANCEMENT]: prevents DB to start up on old version of posting list (magic number)

[rderbier]

What you wanted to do.

v21.12 PR https://github.com/dgraph-io/dgraph/pull/8032

introduce a magic number to prevents DB to start up if the version is not compatible. Avoid corruption.

[mangalaman93]

I think we should have test for this that in case a customer starts the wrong version of Dgraph on a p directory, it shouldn't do bad things. If the tests conclude that we need a magic number, we can cherry-pick this PR. This one should be done soon.

[github-actions[bot]]

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